Plenarvorträge - DPG-Tagungen

Oberflächenphysik Freitag
O 43 Adsorption an Oberflächen IV
Zeit: Freitag 11:15–13:15 Raum: H36
O 43.1 Fr 11:15 H36
Multilayer Adsorption and Wetting of Acetone on Graphite —
•Frank Kruchten — Universität des Saarlandes, Technische Physik
7.3, 66123 Saarbrücken
Adsorption/desorption isotherms of acetone on highly oriented pyrolytic
graphite have been measured by ellipsometry for temperatures
above the bulk triple point. The behavior in the monolayer and submonolayer
regime is conventional, with 2D gas-liquid and 2D liquid-solid
coexistence regions. Further liquid monolayers grow on top of the completed
monolayer. The growth is basically layer-by-layer. For temperatures
between 190 K and the triple point a prewetting-type transition
occurs with a thin-thick jump of the layer thickness on adsorption but a
layer-wise removal of the film on desorption. In this temperature regime
the first monolayer is solid and its molecules are oriented perpendicular
to the substrate whereas the higher layers are orientationally disordered
polar liquid.
This work has been supported by the Deutsche Forschungsgemeinschaft
(Project No. Kn 234/9).
F.Kruchten and K.Knorr, Phys. Rev. Lett. 91, 085502 (2003)
O 43.2 Fr 11:30 H36
The on-surface ordering behavior of O/Rh(111) and H/Pd(111)
studied from first-principles — •Cesar Lazo 1 , Frerich Keil 1 ,
Karsten Reuter 2 , and Matthias Scheffler 2 — 1 TU Hamburg-
Harburg, Eissendorfer Str. 38, 21071 Hamburg — 2 Fritz-Haber-Institut,
Faradayweg 4-6, 14195 Berlin
We study the mesoscopic ordering behavior of simple adsorbates from
first-principles by parametrizing a lattice gas hamiltonian (LGH) with
density-functional theory (DFT) data. Based on this LGH, subsequent
Monte-Carlo Simulations provide the (T,p)-surface phase diagram, which
enables us to systematically discuss the potential and limitations of
this approach by comparing with corresponding experimental data. For
O/Rh(111) the rather large binding energy variations with coverage lead
to a rapid convergence of the LGH expansion. The subtle energy differences
involved in the hydrogen bonding at surfaces on the other hand
provide a critical test case. Only after including more than 15 lateral,
up to quattro interactions we obtain very good overall agreement with
the experimental phase diagram for H/Pd(111), reproducing the critical
temperatures of the two ordered overlayers within 20K.
O 43.3 Fr 11:45 H36
Activated adsorption of ethane on Pt(111) studied by in-situ
high resolution XPS — •T. Fuhrmann, M. Kinne, J.F. Zhu, B.
Tränkenschuh, R. Denecke, and H.-P. Steinrück — Physikalische
Chemie II, Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen
As less reactive basic materials used in chemical industries, alkanes
need in general catalysts to reduce the activation energies for dissociation.
The first elementary step is the adsorption on the catalysts surface.
Under UHV conditions, alkanes show a strong kinetic energy dependence
of their sticking probability on metal surfaces. Using the combination of
a molecular beam and high resolution XPS at the synchrotron facility
BESSY II, we study the species evolving during the adsorption process.
Time resolved measurements allow to determine the adsorption kinetics.
Depending on the surface temperature and the parameters of the
impinging molecules, both molecular as well as dissociative adsorption
channels are observed. For direct dissociation the same surface species is
observed for all beam parameters used. Using temperature programmed
XPS (TPXPS), the thermal evolution of the adsorbed species is studied.
Before reaching pure carbon by complete dehydrogenation, different
intermediates are identified by their C1s binding energies and their vibrational
fine structure. Supported by the DFG (STE 620/4-2).
O 43.4 Fr 12:00 H36
Nitriding a Gold(110)(1x2) Surface — •P. Schulz 1 , M. Gottfried
2 , and K. Christmann 1 — 1 Inst. f. Chemie, FU Berlin, Takustr.
3, 14195 Berlin — 2 Dept. of Chem. Eng., Univ. of Washington at Seatle
USA
Molecular N2 does not adsorb dissociatively on Au surfaces. Nevertheless,
a study of possible N-Au bonding situations appears to be interesting
(in view of e.g. de-NOX catalysis and semiconducter contacting). So far,
the only known research to produce N atoms on Au sufaces at 300 K used
N + ion bombardment techniques; the N species formed was characterized
by XPS (N 1s core level shifts) [L. Siller et al., Surf. Sci. 513 (2002) 78].
In our combined LEED, UPS, TDS and ∆Φ study we physisorbed N2
on a Au(110) surface at ∼ 30 K and exposed it to 500 eV electrons. We
succeeded in activating the molecules to dissociate into N atoms which
are strongly held on Au(110), desorbing around 450 K (physisorbed N2
desorbs below 50 K). We will communicate data on the energetics and
kinetics of the various Nad species on the Au(110) surface.
O 43.5 Fr 12:15 H36
A HREELS, TPD and LEED investigation of crotonaldehyde
on Pt(111). — •Alexander Krupski, Jan Haubrich, Conrad
Becker, and Klaus Wandelt — Institut für Physikalische und Theoretische
Chemie, Wegelerstrasse 12, D-53115 Bonn, Germany
TPD studies of crotonaldehyde adsorbed on Pt(111) at 100K were performed
for masses between 1 and 100 amu as a function of exposure. Only
fragments of the masses 2, 28, 39, 41 and 70 were detected up to 800K.
The results show the desorption of molecular crotonaldehyde at 175K
(monolayer), 164K (2nd ads. state) and 143K (multilayer). A very weak
signal is observed for propylene fragments (39, 41) around 335K. A signal
at 28 amu is seen around 395K, saturating already below the monolayer
exposure. Two signals are detected for molecular hydrogen desorption
around 291K and 425K. The HREEL spectra measured between 100K
and 175K show a close relationship to the gasphase data. By contrast, the
irreversibly adsorbed species, remainig above 175K, shows besides some
shifts sizeable changes in scattering intensities. Vibrations of molecular
CO evolve between 300K and 350K in the HREEL spectra, indicating
the decarbonylation of the surface species. Above 375K only traces of
hydrocarbon species can be observed by HREELS.
O 43.6 Fr 12:30 H36
Water on Ru(001): Interfacial structure investigated by vibrational
spectroscopy — •C. Frischkorn 1 , D.N. Denzler 2 , R.
Dudek 2 , S. Wagner 2 , M. Wolf 1 , and G. Ertl 2 — 1 Freie Universtät
Berlin, Fachbereich Physik, Arnimallee 14–16, 14195 Berlin — 2 Fritz–
Haber–Institut der MPG, Faradayweg 4–6, 14195 Berlin
Vibrational spectroscopy with sum-frequency generation (SFG) is intrinsically
surface/interface sensitive and thus can provide information
on the local binding structure in adsorbate layers. In this contribution,
we present results on the water structure on ruthenium (D2O/Ru(001))
obtained from SFG spectroscopy. The molecular structure of the first
bilayer of this system has been controversially discussed. LEED (low
energy electron diffraction) studies revealed an almost coplanar arrangement
of the oxygen atoms leading to a vertically compressed bilayer [1].
Recent DFT (density functional theory) calculations found that only a
half-dissociated bilayer wets the surface [2] resulting in a coplanar O
geometry which consequently explains the LEED results. Our SFG vibrational
spectra on D2O/Ru(001) taken at various thicknesses, however,
clearly disagree with both structures proposed. Therefore, we suggest a
D2O bilayer of intact water molecules whereby every second molecule
undergoes a hydrogen-metal bond giving rise to a so-called ”hydrogendown”structure
[3].
[1] G. Held, D. Menzel, Surf. Sci. 327, 301 (1995).
[2] P. J. Feibelman, Science 295, 99 (2002).
[3] D. N. Denzler, C. Hess, R. Dudek, S. Wagner, C. Frischkorn, M. Wolf,
G. Ertl, Chem. Phys. Lett. 376, 618 (2003).
O 43.7 Fr 12:45 H36
Semi-Empirical Potentials for Studying Catalytic Processes ?
— •Bernhard Lehner, Karsten Reuter, and Matthias Scheffler
— Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg
4-6, D-14195 Berlin
Ab initio electronic structure calculations like density functional theory
(DFT) significantly contribute to the understanding of catalytic processes
at solid surfaces, but are currently limited to microscopic system sizes
and time scales. A promising approach to reach mesoscopic and macroscopic
regimes is to use semi-empiric potentials, parameterised from ab
initio calculations. The main challenges in the development of such an
intermediate potential are high accuracy and reliability.
We check the suitability of Modified Embedded Atom Method